1. Field of the Invention
The present invention relates generally to a fuel cell utilizing solidous electrolyte. Specifically, the present invention relates to a fuel cell which is composed of stacking solidous electrolyte thin film and a process for forming same.
2. Description of the Background Art
Fuel cells, such as plate type fuel cells are well known cell products utilizing porous substrates.
Generally, fuel cells for the production of electrical energy from a fuel and oxidant are well known in the art. Simply put, such cells are composed of a plurality of cell unit structures comprising sequentially stacked porous substrate, an anode electrode film, an electrolyte layer and a cathode electrode film, the cell units are connected in series. Hydrogen gas as fuel is provided in the cathode side of the fuel cell body and air (Oxygen gas) as oxidant is provided in the anode side thereof, then hydrogen and oxygen are reacted for producing electromotive force and water as a by-product. The electrolyte can be a solid, a molten paste, a free-flowing liquid, or a liquid trapped in a matrix. This invention is concerned with the solid type of electrolyte which is preferred for many applications.
It is also well known in the art that voltage dropping in a solidous electrolyte may be indicated by following formula: EQU V=iRT.times.10.sup.-4
wherein
V: voltage dropping PA1 i: electric current (A) PA1 R: resistance (ohm.multidot.cm) PA1 t: thickness of solidous electrolyte (.mu.m). PA1 R: gas constant PA1 T: absolute temperature PA1 F: Faraday constant PA1 P.sub.1,P.sub.2 : partial pressure of oxygen at both sides of the solidous electrolyte.
From the above relationship, voltage dropping of fuel cell can be reduced corresponding to reducing the thickness of the solidous electrolyte. On the other hand, as a solidous electrolyte is formed on a thin electrode film over the porous substrate, the thickness of the electrolyte is determined by its coverage ability but must be thick enough to support a stacked cell structure, therefore, thinner is better but the minimum thickness is restricted. In view of these points, it has been assumed that thickness in a range of 10 to 50 .mu.m is preferrable for a solidous electrolyte layer.
Porous substrates generally in use have dispersed void sizes in a range of 0.5 to 40 .mu.m. Numbers of pin holes are formed in both the electrode film and the solidous electrolyte where it is stacked on portions of the substrate having relatively large void sizes, when the thin electrode film with the solidous electrolyte are stacked on the substrate.
As well known in the art, electromotive force is produced at both sides of the solidous electrolyte when a concentration difference is created by a partial pressure difference of oxygen at each side. That is, the partial pressure difference forms a kind of concentration cell. The electromotive force of a fuel cell may be derived by the following formula: EQU E.sub.O =(RT/4F).times.ln(P.sub.1 /P.sub.2)
wherein
As shown in the above formula, electromotive force E.sub.O proportionally increases corresponding to the ratio of the oxygen partial pressure. Therefore, any pin holes formed in the solidous electrolyte reduces the partial pressure difference of oxygen to zero, it causes loss of electromotive force or may even make the production thereof impossible. Avoid the formation of pin holes is greatly important for forming fuel cells.